For safer traveling of a vehicle, it is important to reduce blind spots of a driver, and also to quickly detect obstacles such as other automobiles, pedestrians, or bicycles around the vehicle, particularly, in front of the vehicle and to notify the driver of the obstacles in low visibility such as at night or in heavy weather. In recent years, various studies have been made for realizing a visibility assist apparatus that assists such visibility in driving.
With the visibility assist apparatus, if all objects around the vehicle are detected and notified to the driver, excessive information will given to the driver, which will interfere with attention to driving. Thus, a method for optically identifying an object has been studied so that only an object that is to be watched can be notified to a driver.
In order to optically identify an object, an emitted light or a light reflected from the object needs to be precisely extracted, and there is a need for a light measuring apparatus that obtains a spectrum of an environmental light applied to the object.
Although a method for obtaining a spectrum of an environmental light is not disclosed, for example, Japanese Patent Laid-Open No. 2001-66254A describes a method for removing background light such as an illumination light or an outside light in order to measure the strength or the like of a detected light (emitted light such as fluorescence).
Japanese Patent Laid-Open No. 2001-66254A describes a method for removing the background light by performing detection measurement for measuring a light including a detected light in the presence of two background lights Fa(t) and Fb(t) having different cycles, and by performing reference measurement for measuring light that does not include the detected light with time interval Tk set so as to be an integral multiple of a least common multiple of cycles fa and fb of the background lights, and subtracting measured value Sps obtained by the reference measurement from measured value Spc obtained by the detection measurement.
As described above, in order to optically identify an object, an emitted light or reflected light from the object needs to be extracted, and thus a spectrum of an environmental light applied to the object as a noise light needs to be obtained. At this time, if the spectrum of the environmental light cannot be stably obtained, a spectrum of the object cannot be stably obtained.
For example, in the case of an aircraft in the air where there are no objects, that block environmental light, around the aircraft, a light receiving sensor can be provided in an upper portion or the like of an airframe to directly measure a spectrum of the environmental light. However, for a vehicle traveling on a road, there are various artificial objects or natural objects therearound, and the spectrum of an environmental light momentarily changes. Thus, it is difficult to stably obtain a spectrum of environmental light by a direct measuring method.